The most sure fire way to make sure game genie codes work is to just use official Galoob ones. If you're looking at gamefaqs for codes and they are custom codes, some of those might not work.
Yeah this is running Super Mario Allstars + World (PAL) on an NTSC console.
Well us, the community, can help you test things out though. It's good to see progress on this. Kinda strange that the codes and fixes would cause the ROM to either not count down or partially count down loading and then freeze the console. I'm not sure why that is. It almost makes it seem like the menu is checking the ROM before it is sent to the RAM, but I don't know why it would even do that.
Oh and I haven't tested adding my own codes in to the menu yet. I am assuming that works though and that 0.23 probably had no issues. I do like the idea of built in codes better though and it makes the unit more portable that way.
It was interesting to see no codes for Wild Guns in there yet

Since there's a few videos of this game running on this cart. But maybe the PAL one has them, I can try it out
